msm: camera: fd: Avoid integer overflow on position corner case

There is posibility to get negative left/top result
during face coordinate translation if face size is bigger
then center x/y coordinates.

Change-Id: Iab6b36befa3862220ca3723a34e2e9210485e97d
Signed-off-by: Gjorgji Rosikopulos <grosik@codeaurora.org>